Air density computer



July 12, 1960 J. C. ELMS ET AL AIR DENSITY COMPUTER 2 Sheets-Sheet 1 Filed Oct. 12, 1953 n 07.. R R m 5 m m 5 mm L H u V. Cw 5 H H sm H Mm mm W Mm w mm mm on em m y S QE 550.28 55128 $835 5 Sw m |||..v .623 .l.||| wmawmmE 12 M55 E 9.25 PII L a N 538725 mu. zoo fllTlI @335 2 :95 F pzmmmmmta q 7 i l W Q ATTORNEY July 12, 1960 J. c. ELMS ETAL 2,944,736

AIR DENSITY COMPUTER Filed Oct. 12, 1953 2 Sheets-Sheet 2 MACH N2.

fi INVENTOR.

JAMES c. suns 6 BY FREDERICK H. GARDNER v v muw/u v ATTORNEY 7 her computer;

United sates Pate -i 2,944,786 7 AIR DENSITY COMPUTER James C.'Elms, Newport Beach, and'Frederick-H. Gardner, Long Beach, Calif., assignors to North American Aviation, 'Inc.

Filed oer. 12, 1953.8. No. 385,364 7 'Claims. Cl. 235-151) .Q'fThis" invention pertains to means for producing a vo1t-' ragejproportional'to the ratio of the air density surround- -ing a moving vehicleto astandard;air-fdensity, andmore.

-particularly to means for obtaining a voltage proportional tonthe ratio of'the airdensity. surrounding a moving air icraft :at any altitude to a standard density of air. at sea 'leveL. 1 IIt -is necessfaryfor the p-roper operation-of fire control .autopilots to provide avoltage which is proportional to ithe air 'density'surrounding a moving aircraft with re- =spec'trto'la standard air density, which :is usually-taken toibe thedens'ity 'measuredat lea level. The airdensity imist be computed from information-which is available iupon a 'mo'vingcraft): Available upon a moving aircraft for making this computation are ram pressure, staticfipressure, and stagnation temperature. These, parameters sarezimturnzused to compute Mach numberfand, true -velocity. x-This invention contemplates a device-for mab ing this coiriputation iavith:the accuracy' required for the -'prediction or the 'flightT-pa'th offrockets launched from -the ii10ving aircraft and fortcontrolling the sensitivity *fof servo systems connected to, operate the control surfaces fo'f said craft." The computer of this inventionris set' forth inthe specification of application 1 Serial -No..;371,756,'

maximums: 3, 1953, for fIntegrated'Aircraft' Fire Comtrol Autopilot by John R; Moore and David. G; Soergel. Itis' therefore an object OfithlSiIlVGIltiOn tosprovide' an improved airdensity 'computer.'%.-;,. r a i lt is anothe'r object of this invention 'to .providex-means for producing an electrical signal1 proportio1ia1 to' the density of air surrounding amoving vehicle.

It is another object dfthis invention to provide means gfor gprgducing an electrical signal proportional 1:9 the .aatio of thea i r densit-y surrounding a moving vehicle "to a1standard density of-airat'sea levela It; is stillfanother object of this invention t prOvide -means: for creating 'a; voltage proportional to, the ratio of ethe air density? surrounding a-moving ;veh ic1e;-to a standwar v air: en ty-"rv e Itiis; .yet'another object of this invertt ion,toepmvi de tional to static pressure or proportional to the. difference? between'ram and static :presstlre;

, Fig. 4 is a schematic drawing of the device shown in 60 p ,.-I. p parameters of the device of this invention are obtained;

Patented July .12, 1960 2 And Fig. 6 is a schematic drawing of a typical triie 'air speedcomputer. ln Fig. 1, differential .press'uretransducer -1 generates a shaftrotation proportional to the difierencefbetween 5 ram and static pressure measured upon a moving vehicle. Pressure transducer 2 generates a shaft rotation proportional to the static pressure measured upon the moving vehicle. The output of difierential pressure transducer 1 is connected to the input of air density com- :puter 3 and Mach number computer 4. The output of static pressure transducer 2 is connected to the input of -Mach number computer 4. The output ofstatic pressure transducerZ is connected to the input of Mach number ;computer 4, The-outputof Mach number computer 4 -is*a5shaft rotation proportionalto the Mach number of the vehicle. The output of Mach number computer 4 is conuected to the-input of :air density computer '3 and true airspeed computer 5. True air speed computer 5 sproducesa shaft rotation which is proportional to the true air speed of-the vehicle upon which his mounted and is connected to the inputof air density computer 3.

' -In-Fig. 2 voltagesource-6 is connected across resistors ;7 and-8 'in'series; Resistor 8 ;is connected as a linear 2 potentiometer; Resistor 7 is connected as-a variable resistor which has its resistance distributed so that-the 'total' resistance measured across its terminalsis always Zproportidnal -to a predetermined function of it shaft rotation. This function is 30'; J'B B I Q 5 4+40 i600 fwherein BTis the's'haft'rotation. I

The shaft of ire'sistor 7 ismechanically connected to be Fdrivenby' Mach number computer 4. The -shaft of potentiometer 8 'is mechanically *conencted to be driven by differential pressure transducer-1. The movable arm of pot'entiometer'fi is electrically connected through Sum-' r'nin'fg' resistor-'9 tothe input'of amplifier 10. "Potentiom- --eter- 11 is-jconnected "across the output of amplifier 10. "'Potentio'mete'r 11 is a potentiometer whose resistance is distributed sothat theresistance between one fixed and the movable terminal is proportional to the'square of 3th onofits sh'aft'. 'I-he;shaft' of potentiometer 11 1 mechanically connected to thei'outputof true air speed computer- 5. The'movable arm ofpotentiometer 11 "is l eieetrieauy connected through'summing resistor 12 to'the ifnpu't of amplifier 10 to provide a'feedback-loop about amplifier 10. The'total resistance across the terminals of resistor 7 is very mu h larger, preferably of the fiiied'r r minais 'of po'tentiometer s. I Differential pressure transducer 1 and static pressure transducer Z niay *be "of the typ'e set forthin application 'Srial 1%3256374, filed January 14, 1952, now Patent 7 The closed-cycle transducer described: herein i x "rioiliial force balance principle. This means that theoiitp'ut of-the 'trausduceris no longerdependent on the amount of displacement of the pressure aifitai'n- "the element in-its balanced or zero ese fo'rc'es is produced in the'pressure felen'ieht-by' the'i1iput' pressure; Theother force dii'c'e'd" a ystem ef feedback from the output. Kill as f which "might be or the variable fictive 'fE .ipick-ofi type, is employed to y partiire "ef' 'thje pressure sensitive element fioin'its or'mal balanced position. When the element is imbalanced asignal'is sent from; the null "detector-network man amplifier circuit which: actuates;;a;torquing.

o'riier'fof te'n times,-than.the total resistance acros'sthe rielg Hl iqia'rdnerfor Closed Cy'cle Pressure .Trans- I ele *efit, but "depends on a balancing of two recting factor.

device, such as an electromagnetic torquer or motor.

anced condition. Part of the output of the torquing, de-

vice is fed back to the pressure sensitive element as a cor- When the feedback is sufl'icient to, restore the force balance, the torquing device ceases to move. The desired-variation of the current, voltage, resistance inductance, or shaft rotation output is obtained from the output of the torquing device or is obtained from the output of the amplifier circuit, since, the amplifier output current. must be sufiicient to maintain the required torque 'in the electro-magnetic torquing device or motor.

' Referring to Fig. 4 and to the sectioned view shown in 'Fig. 3, the desired output is a variation of resistance in' potentiometers 13 and 14 which is proportional to the difference in the pressures supplied at fittings 15 and 16.

Casing 17, Bourdon tube assembly 18, and plate assembly 19 enclose a pressure-sealed chamber consisting of interconnected chambers 20, 21, andZ-Z. All external electrical connections to the transducer are made through terminals 23 of pressure-sealed socket 24. Fitting 16 is provided in plate assembly 19 to provide a port whereby any desired pressure is impressed in chambers 20, 21,

and 22, and hence one side of Bourdon tube 25. Fitting '15 is attached to Bourbon tube assembly 18 to provide means for exerting input pressure to the other side of the Bourbon tube 25 or to evacuate that side if a static pressure input is desired.

Torsional straight line Bourdon tube 25 is sensitive to changes in the pressure differential between fittings 15 and 16. The movable or torque end of Bourdon tube 25 is rigidly connected to armature 26 of null detector 27. Feedback shaft 28 is also attached to armature 26. Torsional spring assembly 29, consisting of straightline torsional spring 3t) secured to spring collar-s31 and 32, transmits torque to feedback shaft 28 by means of shaft slots 33 and 34. Slot 33 permits axial motion of the torsional devices while transmitting all torque.

Normally the torque of torsional spring 29 is equal and opposed to the torque of Bourdon tube 25 and armature 26 is in a balanced or zero position. A change in the pressure differential upsets this balanced torque position. The unbalanced torque causes balanced armature 26 to rotate about the center line of'Bourdon tube 25. Rotation of armature 26 unbalances the null detector 27. Null detector 27 is inductive E pick-off type consisting of a stator and armature 26. When unbalanced, the null detector 27 sends an error signal out through terminals 23 to'power amplifier 35. In response to the error signal, .power amplifier 35 furnishes power to motor 36. The direction of rotation of motor 36 depends on the phase of the detector error signal.

Motor 36 drives gear'train 37. Output shafts of gear train 37 include the shafts of potentiometers 13 and 14. The shaft of either potentiometer 13 or14 can be extended in length to drive any number of resistors or otentiometers. A further output shaft of gear train 37 is feedback drive shaft 38. Torsional spring collar 32 is secured to feedback drive shaft 38 by taper pin 39. Feedback drive shaft 38 is driven in such a direction as to cause torsional spring 30 to exert a torque on balanced armature 26, tending to return armature 26 to its zero balanced position.

:'and 14 being geared to motor 36' have turned through an angle which is proportional to change in pressure.v "Detail 40 isa spring stop device to prevent damage to the followup spring 30.

The device used in the example described above for obtaining shaft rotation proportional to pressure operates on the principle of maintaining zero deflection of the movable end of Bourdon tube 25. Only a small amount of deflection starts the feedback torque cancelling out the deflection. Thus, the amplitude of deflections is kept very small. Since the magnitude of the hysteresis error of the device depends on the amplitude of deflections from zero position, the hysteresis error is negligible.

In difierential pressure transducer 1, ram pressure is introduced at fitting '15 and static pressure is introduced at fitting 16. In static pressure transducer 2, static pressure is introduced at fitting 16, and the chamber at which fitting 15 is attached is evacuated and sealed oif. In each case the rotation of shaft 38 is a'measure of the pressure diiferential between fittings 15 and 16.

Mach number computer 4 is shown in Fig. 5. I Voltage from voltage source 41 is applied across a'Wheatstone bridge comprising resistors 42, 43, 44', and 45. Variable resistor 42 is mechanically driven by static pressure transducer 2 so that the resistance of resistor 42 is proportional to the static pressure. Resistor 43 is driven by differential pressure transducer 1 so that the resistance of resistor 43 is proportional to the difference between ram and static pressure. resistor which is connected to be driven by motor 46. Motor 46 is driven by amplifier 47 which is connected across the output of the Wheatstone bridge of resistors 42, 43, 44, and 45. The nonlinearity of the resistance of resistor 44 is adjusted to null the bridge when the shaft rotation of motor 46 is proportional to the Mach number of the aircraft. i

True airspeed computer 5 is shown in Fig. 6. Voltage source "48 is applied across the input of a Wheatstone bridge comprising resistors 49, 50, 5 1, and 52. The resistance of resistor 52 is proportional to the stagnation temperature of the air. Variable resistor 50 is mechanically driven by Mach number computer 4 so that the resistance of resistor 50 is a predetermined function of the Mach number. Amplifier 54 is connected across theout- .put of the Wheatstone bridge of resistors 49, 50, 51 and 52 to drive motor 53 with a shaft rotation which is proportional to the true air speed; The nonlinear resistance of nonlinear resistor 49. is predetermined to cause the shaft rotation of motor 53 to be proportional toV;.

The dynamic pressure of a flowing gas into a pitot tube is given by the expression By solving algebraically for p,

2A P ave and LI: 2AP 7P0 fc l flo wherein po is the density of air atsome standard elevation, for example, sea level. r I

Resistor 44 is a nonlinear variable T ZH s f T a In Fig. ,2 n designer resistor 1, is mined in proportion to'nthe Mach number ,of the movinglvehicle f'amegiito se squats upon which itis mounted, theresistance across the terfJr which in turn is proportional'to p and, by choice of appropriate constants is proportional to Thus, a new computer has been created for generating a voltage proportional to the ratio of the air density of static air surrounding a moving craft to the density of air at a predetermined standard such as the density of air at sea level. i Although the invention has been described and illusii 'g'aivoltage proportional'lto said difference between and static pressure divided by said compressibility factoi'; velocity computer means, including a shaft, for producing a shaft rotation proportional to the true velocity of; said' vehicle; analog computer means connectedbe tween said velocity computer means and said potentiometer; means to divide the voltage of said potentiometer m'eans'by thesqu'are of said velocity to thereby generate a voltage proportional'to the air density surrounding said moving vehicle.

3; Means for obtaining a voltage proportional to the ratioof the air density surrounding a moving vehicle to the density of air at sea level comprising a first and. second electricalpotentiometer connected in series with a a voltage soure; means connected to drive said first poten-;

' tiorheter'to'causetheresistance thereof to be proportional to" the compressibility factor; meansconnected to said second potentiometer to drive the arm thereof in proportion-to the difierence between ram and static pressure arm of said third potentiometer being connected to the input of said amplifier to thereby divide said ratio of difference in pressure to compressibility factor by the square of said true velocity to create an electrical voltage'proportional to the ratio of the air density surroundtrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. Means for computing air density surrounding a moving vehicle comprising means for producing a signal proportional to the Mach number of said vehicle compressibility factor computer means responsive to the output of said Mach number means for computing the compressivelocity computer means responsive to a stagnation tem perature input signal and said Mach number producing means for computing the true velocity ofsaid vehicle; and means responsive to said compressibility factor computer means, said differential pressure computer means, and said velocity computer means for dividing said differential pressure by said compressibility factor and by the square of said velocity.

2.. Means for obtaining ratio of the air density surrounding-a moving vehicle to the density of air at sea level comprising Mach number computing means, including a shaft, for producing a shaft rotation proportional to the Mach number of said vehicle; transducer means, including a shaft, for producing 'a shaft a voltage proportional to the rotation proportional to the difference between the ram 7 v and static air pressure measured upon said vehicle; electrical resistor means connected to be responsive to the ing said vehicle to the density of air at sea level.

4. Means for obtaining a voltage proportional to the ratio of the air density surrounding a moving vehicle to the density of air at sea level comprising a variable resistor, shaft means connected to vary the resistance of said resistor, said variable resistor having its resistance distributedso that the total resistance thereof is a compressibility factor function of the rotation of said shaft means, means connected to said shaft means of said resistor to providea rotation of said shaft means proportional to the Mach'number of said vehicle, a first potentiometer'having a shaft and a slider, and a voltage source connected in series with said resistor, means connected to rotate. said shaft of said potentiometer by an amount proportional to the difference between ram and static pressure measured upon said vehicle, a feedback amplifier connected by its input through a summing resistor to said slider of said potentiometer, and means elec trically connected betweenthe output and input of said amplifier for providing feedback proportional to the square of the true velocity of said vehicle whereby the voltage at the output of said amplifier is proportional to the ratio of the air density surrounding said vehicle to a standard density of air at sea level.

. 5. Means for determining the density of undisturbed air in the vicinity of a vehicle moving rapidly through said air-comprising adifferential pressure transducer for promeans responsive to the output of said Mach number.

means for producing a signal proportional to the true airspeed ofsaid vehicle, and means conductively connected to said differential pressure transducer, said Mach number means, and said true airspeed means for producing a signal proportional to the density of said undisturbed air.

6. Means for determining air density in the vicinity of an aircraft comprising differential pressure means for producing a signal proportional to the difference between" static pressure and ram pressure input signals, means responsive to a static pressure input signal and the output 7 of said'difierential pressure means for producing a signal" proportional to the Mach number of said vehicle, means responsive to a stagnation temperature input signal and the output of said Mach number producing means for producing a signal proportional to the true airspeed of said vehicle, and means responsive to the outputs of said difierential pressure means, said Mach number means, and said true airspeed means for producing a signal proportional to the density of air.

7. Means for computing air density in the vicinityof anaircraft comprising difierential pressure computer means responsive to ram and static pressure input signalsfor computing the difference between ram and static pressure upon said aircraft, means responsive to the output of said differential means and to a static pressure input signal for computing the Mach number of said aircraft, means responsive to the output of said Mach number means and to a stagnation temperature input signal for computing the true velocity of said aircraft, means .responsive to the output of saidMach number means for computing the compressibility factor function of said Mach number, means responsive to the output of 8 said compressibility "factor: means and said differential pressure means for dividing said'co'rnpressibility factor by said differential pressure, m ean s responsive to the output of said true velocity means for computing the square of said velocity, and means responsive to the output ofs'aid dividing means and saidvelocity square means. for dividing the output of said dividingme'ans by the.

Obermaiei- May 27, 1923 2,579,220 Vine Dec. 18, 1951' 2,595,185 Zauderer et Apr. 29, 1952 2,714,309 Redemske Aug. 2, 1955 Eisler. et al. Qct. 25, 1955,

OTHER REFERENCES Electronic Instruments (Graham; ,Holdam and Macrae), published by ,McGraw-Hill Book Co., New York, 1948, pages 55-7-56 relied onr V p r 

